It has been mainly proposed that UWB in which a submillimeter wave band ranging from 22 to 29 GHz is used is utilized as a vehicle-mounted or portable short-range radar (SRR).
It is necessary that an antenna of the radar apparatus used in the UWB have a broadband radiation characteristic, and that the antenna have a compact and thin type planar structure considering the fact that the antenna is placed in a gap between an automobile body and a bumper when mounted on the vehicle.
It is also necessary that the antenna make an exploration with a weak radio wave defined by the UWB, and the low-loss and high-gain antenna is required to suppress useless power consumption such that the antenna can be driven by a battery. Therefore, it is necessary that the arrayed antenna can easily be achieved.
For the purpose of the cost reduction, in the antenna, desirably a feed unit of an antenna element can be produced by a pattern printing technique.
As described above, the frequency band of 22 to 29 GHz is used for the UWB radar. However, the frequency band of 22 to 29 GHz includes an RR radio-wave emission prohibited band (23.6 to 24.0 GHz) for protecting a passive sensor of radio astronomy or earth exploration satellite service (EESS).
In 2002, in Non-Patent Document 1, FCC (Federal Communications Commission of USA) discloses a rule in which average power density is not more than −41.3 dBm and peak power density is set to 0 dBm/50 MHz in the frequency band of 22 to 29 GHz.
The rule also stipulates that an elevation-angle side lobe is decreased from −25 dB to −35 dB every few years in order to suppress radio interference to EESS.
Non-Patent Document 1: FCC 02-48 New Part 15 Rules, FIRST REPORT AND ORDER
However, in order to realize the decrease in elevation-angle side lobe, a dimension is increased in a perpendicular direction of the antenna used in the UWB radar, and it is envisioned that the antenna is hardly mounted in a general passenger car.
Therefore, in 2004, FCC adds a revised rule which is a method independent of the elevation-angle side lobe of the antenna as described in Non-Patent Document 2. In the revised rule, radiation power density of the RR radio-wave emission prohibited band is set to −61.3 dBm/MHz which is smaller than ever before by 20 dB.
Non-Patent Document 2: “Second Report and Order and Second Memorandum Opinion and Order” FCC 04-285, Dec. 16, 2004
A method of turning on and off a continuous wave (CW) from a continuous oscillator using a semiconductor switch is adopted in the conventional UWB radar.
In the method, a large residual carrier is generated due to incompleteness of switch isolation. Therefore, as shown by a broken line of FIG. 18, the residual carrier is evacuated to an SRD (Short Range Device) band ranging from 24.05 to 24.25 GHz which is allocated for a Doppler radar.
However, because the SRD band is extremely close to the RR radio-wave emission prohibited band, there is a serious problem that the interference with EESS and the like cannot be avoided.
In order to solve the problem, there has been proposed a method in which a burst oscillator shown in Non-Patent Document 3 is used as the UWB radar.
Non-Patent Document 3: “Residual-carrier free burst oscillator for automotive UWB radar applications”, Electronics Letters, 28th Apr. 2005, Vol. 41, No. 9
The burst oscillator oscillates only when a pulse is on whereas the burst oscillator stops the oscillation when a pulse is off. Therefore, a residual carrier is not generated when the burst oscillator is used in the UWB radar.
Because any spectrum arrangement can be achieved, the band shown by a solid line of FIG. 18 can be used for the UWB radar, and as a result, the radiation power density can be suppressed to a sufficiently low level in the RR radio-wave emission prohibited band.
However, it is not easy to make the radiation power density 20 dB or more lower than a spectral peak only using the burst oscillator.
In this case, when the antenna has a characteristic in which the gain has a steep decline (notch) in the RR radio-wave emission prohibited band, the UWB radar which satisfies the new FCC rule can be realized by use of a combination of the antenna and the burst oscillator.
The invention is intended to provide an antenna suitable to the UWB radar which has the gain notch in the RR radio-wave emission prohibited band.
First of all, it is necessary that a broadband thin type planar antenna be realized as the antenna satisfying the various requirements.
As the thin type planar antenna, there is well known a so-called patch antenna having a configuration in which a rectangular or circular plate-like antenna element is formed on a dielectric substrate by patterning.
However, generally the patch antenna has a narrow band. In order to broaden the band, it is necessary to use a thick substrate having a low dielectric constant.
The low-loss substrate is required in order to use the antenna in the submillimeter wave band, and Teflon (registered trademark) is well known as such substrates.
However, because Teflon has difficulty in bonding a metal film, there is a problem that it is difficult to produce the antenna, resulting in cost increase.
Therefore, it is considered that a circularly polarized wave or a linearly polarized wave is used in the broadband element antenna necessary for UWB. In the case of the circularly polarized wave, there is an antenna such as a spiral antenna having the good characteristic.
However, the UWB antenna in which the linearly polarized wave is used is necessary because the circularly polarized wave cannot be used in the case of the vehicle-mounted short-range radar including a communication function. The realization of the short-range radar with the communication function is recently being studied.
In the case of the linearly polarized wave, there is a problem that it is not easy to obtain the broadband element antenna.
There is known a dipole antenna called bow-tie antenna as an element antenna of the relatively broadband linearly polarized wave. The dipole antenna is formed of a pair of triangles.
However, in the case where the bow-tie antenna is used as the array antenna, disturbance of the directivity is easily generated due to mutual connection between antennas.
A method of increasing the substrate thickness to about a quarter of a propagation wavelength is adopted in order to broaden the band in the planar antenna in which the dielectric substrate is used, and this method is effective in the case where the antenna is used as a single element.
However, in the array antenna in which the plural elements are arrayed, when the dielectric substrate is thickened, a surface wave propagating along the dielectric substrate surface is excited, which results in a problem that the elements are affected by the surface wave to hardly obtain the desired characteristic.